Just discussed the subject with my father and he just laughed...
He reminded of a little experiment we did nearly 2 decades ago when he started teaching me about engines - I was 6 or 7 years old, and I asked this very same question. My father took 5 liter plastic bottle that he modified with a lighter's ignition, a fuel line and tap, and glued the cap on. He squirted lighter gas in through the line and closed the tap, after which he ignited it. When evrything settled, I noticed the bottle had pulled into itself having less air inside it.

And so I never asked again. . .until now.